The hourglass, also known as the "sand clock", is an instrument used to measure time in ancient China. The hourglass is manufactured in much the same way as a leak, which measures time based on the amount of quicksand leaking from one container to another. This method of using quicksand instead of water is because the cold air in the north of China is cold and the water is easy to freeze.

The most famous hourglass is the “five-round hourglass” created by Zhan Xiyuan in 1360. The quicksand flows from the funnel-shaped sand pool to the sand bucket on the side of the starting wheel, driving the first wheel, which drives the mechanical gears of all levels to rotate. The last stage gear drives the middle wheel that rotates on the horizontal plane. The center wheel has a pointer on the axis, and the pointer rotates on a knurled instrument disc to display the moment. This display method is almost the same as the modern clock. The surface structure is identical. In addition, Zhan Xiyuan also skillfully added a mechanical dialing device to the middle wheel to remind the two wooden men standing on the five-wheel hourglass. Every hour or moment, the two wooden people will come out on their own and drum the time. This hourglass is separated from the auxiliary astronomical instrument and has become a mechanical clock structure independently.

Due to the waterless line system, the hourglass is more accurate than the leak.